Mechanical quantity sensor

Abstract

A mechanical quantity sensor includes a current-to-voltage converter/signal adder circuit that converts electric current signals flowing through two piezoelectric vibrators into voltage signals. The piezoelectric vibrators receive stresses generated by a mechanical quantity, such as acceleration, in opposite directions. A voltage amplifier/amplitude limiter circuit amplifies an added signal obtained from the two voltage signals and limits its amplitude. A phase-difference-to-voltage converter circuit detects a difference in the phases of the added signal and a feedback voltage signal applied to an acceleration detection element. A phase shifter circuit controls the phase of the feedback voltage signal so that the phase is a predetermined phase. A filter circuit minimizes frequency components higher than an oscillation frequency in an unwanted frequency band. By increasing the resistance of resistors so as to increase the damping ratio, temperature stability is increased. Accordingly, abnormal oscillation is prevented by the filter circuit, and fluctuation in the characteristics is minimized by the phase control circuit.

1. A mechanical quantity sensor comprising:
 two piezoelectric vibrators arranged to receive stresses caused by a mechanical quantity in opposite directions; 
 a voltage signal applying circuit arranged to apply a voltage signal to the two piezoelectric vibrators; 
 a current-to-voltage converter circuit arranged to convert electric current signals flowing through the piezoelectric vibrators into voltage signals; 
 a phase difference signal processing circuit arranged to detect a phase difference between the voltage signals output from the current-to-voltage converter circuit and output a mechanical quantity detection signal; and 
 resistors arranged in electric current paths of the two piezoelectric vibrators; wherein 
 the voltage signal applying circuit includes:
 a voltage amplifier and amplitude limiter circuit arranged to amplify the voltage of an added signal corresponding to an added value of currents flowing through the two piezoelectric vibrators and to limit the amplitude of the voltage signal output from the voltage amplifier circuit to a predetermined value; 
 a phase control circuit arranged to detect the phase difference between a feedback voltage signal applied to both of the two piezoelectric vibrators and the added signal and control the phase of the feedback voltage signal so that the phase difference equals a predetermined value; and 
 a filter circuit arranged to minimize unwanted frequency components of the feedback voltage signal; and 
 
 wherein the mechanical quantity sensor is oscillated by the piezoelectric vibrators, the voltage amplifier circuit, the amplitude limiter circuit, the phase control circuit, and the filter circuit. 
 
   
   
     2. The mechanical quantity sensor according to  claim 1 , wherein the filter circuit is a low-pass filter having a passing band including the oscillation frequency. 
   
   
     3. The mechanical quantity sensor according to  claim 1 , wherein the phase control circuit includes:
 a phase-difference-to-voltage converter circuit arranged to convert the phase difference between the added signal and the feedback voltage signal into a voltage signal; 
 a comparator circuit arranged to compare an output signal from the phase-difference-to-voltage converter circuit and a reference signal; and 
 an all-pass filter including a voltage controlled resistance circuit whose impedance is changed in accordance with an output voltage from the comparator circuit, wherein the phase of the all-pass filter is changed in accordance with the impedance of the voltage controlled resistance circuit. 
 
   
   
     4. The mechanical quantity sensor according to  claim 1 , wherein the phase control circuit is arranged to control the phase difference between the feedback voltage signal and the added signal so that detection sensitivity of the mechanical quantity is maximized. 
   
   
     5. The mechanical quantity sensor according to  claim 1 , wherein the mechanical quantity is acceleration. 
   
   
     6. The mechanical quantity sensor according to  claim 1 , wherein the mechanical quantity is angular acceleration. 
   
   
     7. The mechanical quantity sensor according to  claim 1 , wherein the mechanical quantity is angular velocity. 
   
   
     8. The mechanical quantity sensor according to  claim 1 , wherein the mechanical quantity is a load. 
   
   
     9. A mechanical quantity sensor comprising:
 two piezoelectric vibrators arranged to receive stresses caused by a mechanical quantity in opposite directions; 
 a circuit arranged to apply a voltage signal to both of the two piezoelectric vibrators, wherein the circuit includes:
 a voltage amplifier/amplitude limiter circuit arranged to amplify the voltage of an added signal corresponding to an added value of currents flowing through the two piezoelectric vibrators and to limit the amplitude of the voltage signal output from the voltage amplifier circuit to a predetermined value; and 
 a phase control circuit arranged to detect the phase difference between a feedback voltage signal applied to both the piezoelectric vibrators and the added signal and control the phase of the feedback voltage signal so that the phase difference equals a predetermined value; 
 
 a circuit arranged to convert electric current signals flowing through the piezoelectric vibrators into voltage signals; and 
 a circuit arranged to detect a phase difference between the voltage signals output from the converter circuit and output a mechanical quantity detection signal. 
 
   
   
     10. The mechanical quantity sensor according to  claim 9 , wherein the circuit arranged to apply a voltage signal to both of the piezoelectric vibrators further includes a filter circuit configured to minimize unwanted frequency components of the feedback voltage signal. 
   
   
     11. The mechanical quantity sensor according to  claim 10 , wherein the filter circuit is a low-pass filter having a passing band including the oscillation frequency. 
   
   
     12. The mechanical quantity sensor according to  claim 9 , wherein the phase control circuit includes a phase-difference-to-voltage converter circuit arranged to convert the phase difference between the added signal and the feedback voltage signal into a voltage signal. 
   
   
     13. The mechanical quantity sensor according to  claim 12 , wherein the phase control circuit includes a comparator circuit arranged to compare an output signal from the phase-difference-to-voltage converter circuit and a reference signal. 
   
   
     14. The mechanical quantity sensor according to  claim 13 , wherein the phase control circuit includes an all-pass filter including a voltage controlled resistance circuit whose impedance is changed in accordance with an output voltage from the comparator circuit, wherein the phase of the all-pass filter is changed in accordance with the impedance of the voltage controlled resistance circuit. 
   
   
     15. The mechanical quantity sensor according to  claim 9 , wherein the mechanical quantity is one of acceleration, angular acceleration, angular velocity, and a load. 
   
   
     16. The mechanical quantity sensor according to  claim 9 , further comprising resistors connected in series to the two piezoelectric vibrators. 
   
   
     17. The mechanical quantity sensor according to  claim 9 , further comprising resistors connected between inputs of operational amplifiers in the converter circuit and ground.